This makes more dependencies lazy. This has a practical effect of
reducing the number of dependencies that need to be downloaded when
running certain zig build steps.
This is all limited because we're blocked on an upstream Zig issue:
https://github.com/ziglang/zig/issues/21525 This prevents us from
fully avoiding downloading many dependencies, but at least they're
relatively small.
One major improvement here is the usage of `lazyImport` for
`zig-wayland` that prevents downloading `zig_wayland` unconditionally on
all platforms. On macOS, we don't download this at all anymore.
Another, weirder change is that all our transitive dependencies are now
marked lazy (e.g. glslang's upstream source) even if the glslang build
always requires it. This was necessary because without this, even if we
simply referenced glslang in the root build.zig, it would force the
source package to download unconditionally. This no longer happens.
